Electroluminescent lamps typically provide approximately 30 foot-lamberts of illumination and are thus suitable for various low-intensity illumination applications, such as decorative lighting, egress lighting, cockpit and dashboard display panels, and membrane switches. They have also been used as backlighting sources for liquid crystal display (LCD) devices. However, most LCD applications, including black/white and color LCD displays and high definition displays, require greater backlighting illumination than electroluminescent lamps can provide. Furthermore, most electroluminescent lamps have poor maintenance characteristics: they typically degrade to about half their initial brightness within 2000 hours of operation.
Fluorescent lamps, which provide between 2000 and 4000 foot-lamberts of illumination, have been used as illumination sources for these LCD display devices. However, when used in LCD display applications, fluorescent lamps have their own disadvantages. For example, they are bulky. Being made of glass, they are also fragile and thus are unable to withstand rugged environments; if broken, they may release small amounts of mercury. They also do not operate at temperatures below -20.degree. C. In contrast, electroluminescent lamps do not have the disadvantages of size and construction that fluorescent lamps have. They are quite small and thin, light in weight, extremely rugged, and they can operate at temperatures well below -20.degree. C.
To be useful in LCD backlighting applications, electroluminescent phosphors must emit in narrow bands of the blue, green and red zones of the visible spectrum. Specifically, the blue emission wavelength should preferably be between 460-470 nm, the green emission wavelength should preferably be between 535-545 nm, and the red emission wavelength should preferably be between 610-650 nm.
One electroluminescent phosphor which meets the above-described blue emission color requirements for LCD backlighting applications is a blue-emitting copper-activated zinc sulfide phosphor, ZnS:Cu. Zinc sulfide phosphors and methods of making them are described in U.S. Pat. No. 2,807,587 to Butler et al., U.S. Pat. No. 3,031,415 to Morrison et al., U.S. Pat. No. 3,031,416 to Morrison et al., U.S. Pat. No. 3,152,995 to Strock, U.S. Pat. No. 3,154,712 to Payne, U.S. Pat. No. 3,222,214 to Lagos et al., U.S. Pat. No. 3,657,142 to Poss, and U.S. Pat. No. 4,859,361 to Reilly et al., all of which are assigned to the assignee of the instant invention. However, notwithstanding their emission color characteristics, none of these electroluminescent zinc sulfide phosphors, nor any other electroluminescent phosphors, are sufficiently bright for use in most LCD backlighting applications or high definition display devices.
It would be an advantage in the art to provide an electroluminescent phosphor having improved luminance and maintenance for use in LCD and high definition display devices, and a method of making the phosphor.